1. Field of the Invention
The present invention relates to estimating a property of an earth formation. More specifically, the invention relates to apparatus and methods for performing transient electromagnetic sounding in a borehole to determine the property.
2. Description of the Related Art
Exploration and production of hydrocarbons requires knowledge of a subsurface earth formation that may contain reservoirs of the hydrocarbons. Transient electromagnetic (TEM) sounding at the surface of the earth is one way of gathering this knowledge.
Traditional TEM sounding is based on applying a series of electromagnetic energy pulses to the earth formation using a switch-off current mode. The series of pulses requires identical sharp pulses of source current applied to a transmitter coil. The pulses induce circulations of current in the earth formation. The circulations of current in turn induce electromagnetic signals in a receiver. A characteristic of the electromagnetic signals can be related to a property of the formation such as conductivity.
Traditional TEM sounding at the surface of the earth can have some drawbacks. One drawback is that earth formations containing reservoirs of hydrocarbons may be located deep inside the earth. The reservoirs may be beyond the reach of the electromagnetic energy pulses. Another drawback is that traditional TEM sounding may not be able to discriminate between different distances into the formation.
Therefore, what are needed are techniques to measure a property of an earth formation deep inside the earth using TEM sounding. Preferably, the techniques can discriminate between different distances into the earth formation.